You want to know how many participants at a convention of the Green Party would like to limit immigration, but worry that some of your interviewees might be reluctant to express their true feelings about the topic.
You decide to use k-anonymity.
What do you give participants in terms of materials. (1 credit)
What do you instruct them to do? (1 credit)

Assume you found that 85% answered 'yes'.
What is the true percentage for 'yes'? (show the math) (2 credits)

Approach

Material: A coin

Instruction:
Flip the coin.
If you have a number on it, always say yes.
If not answer the question correctly.

Considering that ´50%´ answered yes anyway, because they got a number only ´50´ answered the question.
So ´35%´ of ´50%´ said yes, meaning ´35/50 = 70%´.

Solution

Material: A coin
Instruction: Flip the coin. If you have a number on it, say yes. Else answer the question correctly.